So I was able to reproduce the error. Initially I was passing an object method as a handler for an event which resulted in the strange NaN, Undefined behavior.... start() { this.showDevTools() this._iteration = new Iteration(this, 2) GDT.on(GDT.eventKeys.gameplay.weekProceeded, this._iteration.weekPasses ); } ...

You will notice that the handler is passed with a reference to this however this keyword is always easy to trip on. If we assign the value of this to the parent scope in a variable named _this then wrap the weekPasses invocation in a function, we can get the desired behavior.